Method for balancing pressure applied to film-forming material in the deposition thereof onto a casting surface



SURFACE 3 Sheets-Sheet 1 J y 2 Rd. METZ. JR

METHOD FOR BALANCING PRESSURE APPLIED TO FIL IN THE DEPOSITION THEREOF ONTO A CAST Filed July 51, 1967 mm M July 21, 1970 P. J. METZ, JR 3,520,964

FORMING MATERIAL METHOD FOR BALANCING PRESSURE APPLIED TO FILM IN THE DEPOSITION THEREOF ONTO A CASTING SURFACE Filed July 31, 1967 5 Sheets-Sheet 2 INVENTOR. Pin-'4 J3 M572, J'e BY a July 21, 1970 P. J. METZ. JR 3,

METHOD FOR BALANCING PRESSURE APPLIED TO FILM-FORMING MATERIAL IN THE DEPOSITION THEREOF ONTO A CASTING SURFACE 3 Sheets-Sheet 5 Filed July 31, 1967 PuMP 50:71am PuMP INVENTOR. P5754 J MIT'Z, Jz

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IrraI/vi/ United States Patent Office 3,520,964 Patented July 21, 1970 3,520,964 METHOD FOR BALANCING PRESSURE APPLIED TO FILM-FORMING MATERIAL IN THE DEPOSI- TION THEREOF ONTO A CASTING SURFACE Peter J. Metz, Jr., Castro Valley, Calif., assignor to Crown Zellerbach Corporation, San Francisco, Calif., a corporation of Nevada Filed July 31, 1967, Ser. No. 657,094 Int. Cl. 132% 17/04; B29d 7/02 US. Cl. 26490 4 Claims ABSTRACT OF THE DISCLOSURE A method for the extrusion of film-forming material from an extrusion device wherein a vacuum is applied to the trailing face of an unsupported section of the film which is continuously deposited onto a continuously moving casting surface. The vacuum applied to the unsupported section is substantially balanced to cause substantially even deposit of the film on the casting surface. The balance is achieved by concentrating application of the vacuum by independent duct openings at the respective ends of the extrusion device so as to withdraw outside air which can enter behind the trailing face of the film adjacent the ends of the film.

BACKGROUND OF THE INVENTION This invention relates to the deposit of film-forming material onto a moving casting surface on which film is formed, and more particularly, to effecting even deposit of such material onto the surface by application of a vacuum to the trailing face of the material as it is being deposited, and balancing the application of vacuum across such trailing face.

In the formation of film forming material wherein a stream of the material is continuously deposited onto a continuously moving casting surface from an extrusion device located over such surface and spaced therefrom, a continuous unbroken and unsupported section of the material exists between the extrusion device and the casting surface. To effect substantially uniform or even deposit of the material onto the surface, and also prevent air and smoke generated by hot molten material such as extruded polyolefin from being trapped or occluded between the film being formed on the casting surface and the casting surface, a vacuum applying device, known as a vacuum knife, has been employed to apply suction against the trailing face of the unsupported section and thus allow atmospheric pressure to act on the leading face of such section.

The more even the deposit of the material transversely of the casting surface the better the optical properties and uniformity of caliper (thickness) of the film being formed. However, a problem has heretofore existed in obtaining equalization of the applied vacuum (balancing of pressure) along substantially the entire width of the unbroken section of the deposited stream of material. Partitioning of the vacuum knife to provide a plurality of adjacent suction applying compartments extending across the entire width thereof has been employed in an effort to balance such pressure. However, such type of installations are expensive and require close and frequent damper regulation among the various compartments. Moreover, even in such type of installation variations in pressure among the compartments occurs.

SUMMARY OF THE INVENTION Pursuant to this invention, it has been found that the unbalancing of such pressure is due to leakage of outside air which can enter behind the trailing face of said unbroken film section adjacent each end of the suction device. This leakage of outside atmosphere (atmospheric pressure) at each end of the suction device wherein the suction is applied centrally of the device, causes the extent of vacuum to vary gradiently from each end thereof to the center because at the ends the leakage air is more concentrated and gradually diminishes in concen tration toward the center under the vacuum effect across the entire width of the suction device.

The invention hereof obviates such end air leakage by applying the vacuum by an independent duct structure directly over a relatively short zone adjacent each lateral edge or end of the unbroken film section at each end of the suction device. In other words, the application of vacuum is concentrated "by such duct structures directly at the aforementioned ends, and is not applied directly intermediate such ends because the space between the spaced apart duct structures is in indirect communication with such vacuum source only through the duct structures. This results in substantially all leakage air at said ends in being immediately withdrawn, with consequent automatic balancing and equalization of pressure uniformly along substantially the entire width of the suction device and unbroken film section, and which will remain uniform within practical chill roll operational speeds and vacuum application ranges. In this connection, the specific form of the duct structure is immaterial, as long as the vacuum is concentrated at the ends in the manner described to preclude outside air leakage from flowing inwardly toward the center of the suction device.

From the preceding it is seen that the invention has as its objects, among others, the provision of an improved method for balancing pressure resulting from suction applied to a moving unbroken section of film-forming material being continuously deposited onto a continuously moving casting surface, and which are simple and economical. Other objects will become apparent from the following detailed description and accompanying drawings in which:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a fragmentary end view of an apparatus for extruding film-forming material onto a moving casting surface, and including a preferred form of vacuum knife and end duct structure embodied therein;

FIG. 2 is a fragmentary longitudinal section taken through the duct structure, the plane of the section being indicated by line 2-2 of FIG. 3;

FIG. 3 is a fragmentary tranverse section taken in planes indicated by the line 3-3 in FIG. 2, with a portion of the view broken away to shorten the same, and a portion of the duct structure broken away to disclose the construction more clearly;

FIG. 4 is a fragmentary longitudinal section taken in the plane indicated by the line 44 in FIG. 3;

FIG. 5 is a fragmentary transverse section similar to the section of FIG. 3, illustrating a modification;

FIG. 6 is a section similar to FIG. 5, illustrating a further modification;

FIG. 7 is a fragmentary end elevation of a modification wherein an end wiping pad is employed;

FIG. 8 is a pictorial view in the form of a graph illustrating the pressure imbalance absent the invention hereof; and

FIG. 9 is a similar view, illustrating the balancing of pressure obtained by the instant invention.

DETAILED DESCRIPTION Although the instant invention can be employed in the deposition of any film-forming material onto a moving casting surface for film formation, such as a solventresin or dope-containing coating composition, it is of particular advantage when employed for the extrusion of hot molten polyolefin, such as polyethylene or polypropylene, onto such moving casting surface for imparting optimum optical properties to the film by frost line control, as described and claimed in assignees patent to Aronsen, No. 3,154,608, dated Oct. 27, 1964. The apparatus comprises a conventional continuously rotatable chill or cooled casting roll 2 having a casting surface 3 which for the formation of smooth film should be highly polished, usually chromium plated. Although a casting roll is desirable, any other continuously moving casting surface, such as an endless metal belt may be employed.

A conventional extruder 4 is provided over the casting roll widthwise in spaced relationship thereto. Film-forming material, such as polyethylene, is continuously fed into the extruder under pressure by suitable conventional means (not shown); and the extruder is maintained in heated condition by suitable means (not shown). Continuous flow of the hot molten material from the extruder is through extrusion slot 6. It will be noted that an unbroken section or sheet of unsupported extruded material indicated at 7, exists in the space between extruder slot 6 and casting surface 3, and flows continuously from the discharge end of slot 6 to the location at which the material is deposited onto casting surface 3.

The face 8 of the extruded material away from cast ing surface 3 is referred to as the leading face, and the opposite face 9 adjacent the casting surface is designated as the trailing face. A vacuum applying device 11, namely, a so-called vacuum knife, terminates in a tapered nozzle portion 12 having a pointed front edge 13 terminating closely adjacent unbroken film section 7; the entire vacuum applying device being attached by suitable mounting means 14 to the rear face of the extruder 4. Desirably, the vacuum knife is formed of sheet metal.

Nozzle section 12 of the vacuum knife 11 has an upwardly slanting top wall portion 16 resting against a complementary slanting surface of extruder 4, and which joins an upper wall portion 17, thus providing a closed top. To prevent air from cooling the extrusion die, a thin layer of insulation 16, preferably asbestos about thick, is clamped between wall portion 16 and the rear face of the extrusion die. The respective ends of vacuum knife 11 are closed by end walls 18; and the portion of such walls 18 in the nozzle section are curved at their lower edges 19 to conform to the curvature of casting roll 2. Such lower edges are positioned as close as practical to the casting roll but to obviate interference, a slight clearance 21 is provided; about A", but this dimension is not critical. The bottom of nozzle section 12 may be closed by a bottom wall portion, if desired, but this is not necessary and for simplicity of structure the bottom is open over the casting roll.

From the preceding, it is seen that a chamber for application of suction to unsupported film-forming section 7, is formed over casting surface 3, by top wall 16, 17 and end walls 18 of the vacuum knife. This chamber is open toward unsupported film section 7. Means is provided to seal off such chamber at the rear by a barrier pad assembly indicated generally by reference numeral 22 and which is commonly employed in apparatus of the character described.

In the embodiment illustrated, such barrier pad assembly comprises a bar 26 extending transversely the entire width of the vacuum applying device, and secured to the underside of wall portion 17 by means of bolts 27; the bar 26 also serving to reinforce the vacuum knife. A trackway is formed extending transversely the entire width of the vacuum knife to support slidably for ready endwise removal and replacement, a barrier or wiping pad 28 in frictional engagement with the casting surface 3.

The trackway comprises a plate 29 secured by bolts 27 to a spacer bar 31 and another plate 32 also secured by bolts 27 and having a curved front edge 33 for slidably supporting the front edge of a bar 34 to which pad 28 is secured by any suitable means, such as a suitable adhesive. The rear edge of bar 34 is slidably supported between plate 32 and plate 29. If desired, bar 34 may be of paperboard and stapled to the pad 28 which is advantageously of any suitable absorbent material, such as felt or plush. In addition to scaling off the rear of the vacuum knife, pad 28 maintains casting surface 3 free of oil, as explained in the aforementioned Pat. 3,154,608.

When a vacuum is applied to the unbroken film-forming section 7 to provide optimum optical properties to polyolefin film, as is explained in detail in the aforementioned Pat. 3,154,608, it is seen that although such section 7 acts as a barrier against air leakage at the front of the vacuum knife, and the rear is sealed by the barrier pad assembly 22, air can leak into the vacuum knife behind trailing face 9 of section 7 from adjacent each end thereof through the open end space S between the front of nozzle section 12 and unbroken film-forming section 7. Also, in the embodiment illustrated, air can leak through the end clearance space 21 because the bottom of the nozzle section 12 is uncovered. However, even when the bottom is covered, air will still leak in through the end spaces S.

As a result of such end air leakage, vacuum which has heretofore been applied transversely across the entire width of trailing face 9 of section 7 becomes unbalanced with the greater pressure at the ends of such section, which tapers off gradually toward the center. As a result of such unbalanced pressure, uneven deposition of the filmforming material onto the casting surface occurs. This has the disadvantage of impairing the optical properties of the film formed on the casting surface for the reasons explained in detail in the aforementioned Pat. 3,154,608. This imbalance is pictorially illustrated by curve A in FIG. 8 wherein the ordinate represents pressure. It will be noted that the pressure varies gradiently from each end toward the center along the width of the vacuum knife which is indicated by L.

In the invention hereof, means is provided to concentrate the application of suction directly at each end of the vacuum knife to withdraw substantially all of the leakage air at the respective ends, and thus prevent it from flowing along the entire width of the vacuum knife behind unsupported film section 7. This is done by providing an independent duct structure which forms an independent suction opening adjacent each end of the 'vacuum knife and film section 7 thus creating a separate suction stream for withdrawing outside leakage air directly at each end. Each of such duct structures is connected to a common suction pump which applies the vacuum.

With reference to an advantageous embodiment llllllS- trated in FIGS. 1 through 4, each duct structure 41 comprises an independent tube 42 rectangularly shaped at the bottom portion which merges into an upper cylindrical portion; the tubes being connected to a common suction pump 42. The bottom of the duct structure forms an independent bottom suction zone opening indicated at Z (FIG. 3). Zone Z is of a relatively short length at each end of the vacuum knife for application of a separate suction stream directly at such end. In this connection the length of each vaouum applying zone Z is not dependent upon the width of the vacuum knife or the extruded stream.

Usually in extrusion systems of the character described, the suction device and associated casting roll 2 may vary in width in the order of about 32 to 112 inches depending upon the commercial installation but the suction zone Z can be the same length for all such widths as its prime purpose is to withdraw substantially all end leakage air. This automatically results in balancing of pressure substantially along the whole width of the unsupported film section 7.

The length of each zone Z should be sufficient so as not to create too high a velocity with a consequent throttling effect as this may result in fluttering of film section and yet it should not be too long because then there will be imbalance of pressure too far inwardly of the ends. In this connection, and as explained in the aforementioned Pat. 3,154,608, only a slight amount of vacuum equivalent to about 0.10 to 0.8 inch of water is employed in extrusion of polyolefin film-forming material depending on the character of the material being extruded. Within this range, a suitable length of zone opening Z is about 2 to 6 inches depending on the character of material being extruded.

For 1.0 mil thick (0.001") polyethylene, commonly employed as wrapping material, the vacuum applied is usually equivalent to about .3 to 0.6 inch of water, and a suitable length for zone Z is about 3 inches. In any event, for any particular film-forming material being extruded, the extent of suction applied can be regulated in accordance with the length of zone Z to obtain optimum eiTects.

As a result of the described construction, the application of pressure is balanced and equalized substantially the entire width along the vacuum knife as is illustrated pictorially by curve B in FIG. 9. Only at the relatively short end zones Z to which the suction is applied directly is there a slight imbalance. However, such imbalance is relatively immaterial because the main body or width of the film is uneffected, and moreover, the edges of the film are usually trimmed off.

All the parameters outlined in the aforementioned patent are applicable herein. Although not essential to the invention, but desirable, the vacuum knife is of a width to overlap slightly beyond the side edges of the extruded film, usually about an inch or inch and a half, as can be seen by comparison with the film width W indicated in FIG. 3.

For the purpose of equalizing friction variables in the respective tubes 42, controllable dampers 43 may be provided although they are not functionally necessary. Each damper is mounted on a shaft 44 pivoted in a tube 42 and secured to a slotted sector 46 which can be set in position by means of wing nut 47.

FIG. illustrates a modification which is the same as that described, except that inside vertical baflle plates 51 are provided adjacent the inside of the concentrated suction applying zone. However, these bafile plates lend very little effect.

FIG. 6 illustrates another embodiment wherein the duct structures 52 providing the direct suction openings at the respective ends of the vacuum knife 53 are formed by blocking off the underside of the vacuum knife nozzle by means of an imperforate baflle plate 54 intermediate the ends of the vacuum knife blllt terminating short of such ends. In such arrangement, suction pump 56 is connected to the vacuum knife by a single tube 57 over bafile '54.

FIG. 7 illustrates another modification wherein a pad 61 of suitable soft material, such as felt or plush, is secured to each end of the vacuum knife and engages over the end of the casting roll for the purpose of reducing leakage of air, particularly through clearance space 21, to thus enable. lower vacuum pump speeds. However, leakage is not completely blocked off as it still occurs, particularly through space S in front of the vacuum knife.

What is claimed is:

1. In the method wherein a stream of film-forming material is continuously extruded onto a continuously moving casting surface and suction is applied along substantially the entire width of the trailing face of a section of said stream over said casting surface; the improvement of substantially evenly balancing pressure along said section to efiect substantially even deposition of said stream onto said casting surface, which comprises providing spaced apart duct structures, providing communication of each of said duct structures with a source of vacuum and with said film section along a zone adjacent an end of said section whereby the space along said section between said duct structures at such end zones is in indirect communication with such vacuum source only through such end zones and there is communication between outside air and such vacuum source only at such end zones, and drawing a vacuum thus causing outside air which can enter behind the trailing face of said section at such section ends to be withdrawn through said duct structures and thereby substantially precluding such outside air from flowing inwardly toward the center of said section.

2. The method of claim 1 wherein each zone is of relatively short length along said section, said length being sufiicient to avoid a velocity of outside leakage air which would cause fluttering of said section and at the same time obviate imbalance of pressure substantially along the width of said stream except adjacent the ends thereof.

3. The method of claim 2 wherein the material is hot molten polyolefin and each zone is about 2 to 6.0 inches in length, and the suction applied is equivalent to about 0.10 to 0 .80 inch of water.

4. In the method wherein a stream of film-forming material is continuously extruded onto a continuously moving casting surface and suction is applied along the trailing face of an unsupported section of said stream over said casting surface; the steps of drawing a vacuum from a common source and effecting application of suction substantially along the entire width of said unsupported section of said stream, providing spaced apart duct structures, providing communication of each of such duct stnuctures with said common source of vacuum and with such film section along a zone adjacent an end of said section whereby the space along said section between said duct structures at such end zones is in indirect communication with said vacuum source only through such end zones and there is communication between outside air and said vacuum source only at such end zones to thereby substantially prevent outside air which can enter behind the trailing face of said section from flowing inwardly toward the center of said section, and thus maintain substantially uniform pressure between said end zones for effecting even deposition of material onto said casting surface.

References Cited UNITED STATES PATENTS 2,681,294 6/ 1954 Beguin.

FOREIGN PATENTS 1,359,130 3/1964 France.

ROBERT F. WHITE, Primary Examiner I. H. SILBAUGH, Assistant Examiner US. Cl. X.R. 264216; 1815 

